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The situation at the Fukushima Daiichi powerplant has worsened significantly as it becomes clear that one and possibly two reactors there have suffered a breach in primary containment, making the incident definitely the second worst nuclear accident yet seen.

Nonetheless its human consequences seem certain to remain insignificant against the horrifying backdrop of the earthquake tragedy elsewhere in Japan: and there remain no grounds for anyone to fear for their health.

Nuclear experts at MIT confirm that leaks from the suppression chamber at Daiichi No 2 reactor reported yesterday do in fact amount to a breach in primary containment, contradicting a statement supplied to World Nuclear News at the time by plant operator TEPCO. The doughnut-shaped suppression chamber is believed to have been damaged by a hydrogen-oxygen explosion like those which have wracked the site since the weekend, but this time occurring inside the primary containment shell rather than outside as previously seen.

It now appears that a similar breach may have taken place at the plant's No 3 reactor: Japanese chief cabinet secretary Edano raised the possibility in a briefing during the early hours of today (UK time). There was no loud blast as at No 2, but a brief burst of intense radiation emissions similar to that following the No 2 rupture has been seen at No 3.

The original radiation surge following the No 2 breach prompted withdrawal of most of the hundreds of workers then on site. A small team remained. The initial rise in radiation then declined, but the further surge early this morning led to a brief total evacuation. However, as this is written a small team is back on site. Plant personnel are reportedly spending most of their time in heavily protected control rooms.

According to the MIT experts, these rises in radiation will have been caused mostly by short-lived radioactive isotopes within the cloud of steam and gases emitted from reactor No 2 and possibly from No 3: the bulk of these decay to small levels within minutes. Most of them are isotopes of noble gases such as argon, meaning that they are chemically unreactive and can't be absorbed by human bodies, the ground, plants or animals etc.

As of latest reports, the Nos 1, 2 and 3 reactor cores are still being cooled using seawater pumped by the site's firefighting systems. Their residual heat has now dropped to around 0.5 per cent of normal output, easing the problems of cooling them. The primary power reaction was shut off as control rods were automatically slammed in when the quake hit, but intermediate reactions will continue for months: present levels of heating - estimated to be approximately 12 megawatts at the No 2 reactor - will have halved again by the summer.

If cooling can be maintained, the cores will continue to emit mainly shortlived isotopes, and the radiation problem will remain minimal. If cores melt down fully - it is acknowledged that the fuel rods have already sustained significant heat damage - the potential is there for significant amounts of dangerous long-lived radioisotopes of chemicals such as iodine to escape from the breached primary containment vessel (or vessels), though the residual heating power of the core material would be much reduced by melting and probable resolidification in water remaining at the bottom of the containment, or having dispersed into the steel and concrete base of the vessel.

At present, according to Edano, radiation levels within the plant are back down to levels where there is "on the whole no health hazard".

Elsewhere at the stricken plant there have been fires at a storage pool in the No 4 reactor building, which was shut down when the quakes hit. Spent fuel rods from the reactor are stored in the pool to keep them safely cool. An initial fire, doused by firefighters in the early hours of yesterday morning, may have been caused by an oil leak but there is strong evidence to suggest that hydrogen has emitted from the pool - which would also suggest that the spent rods are exposed to the air.

If spent fuel rods are left exposed long enough they will become so hot as to suffer damage, though this takes some time. Edano stated this morning that efforts to get water into the pool are to begin shortly using pumps on the ground, an earlier plan to drop it from helicopters having been abandoned. Edano said it was important to add water gradually "as there are safety concerns" with dumping a large amount in at once. This would indicate that the rods are believed to be exposed and hot, and a steam explosion could result from a sudden massive water dump.

Asked if it was possible for the spent rods to restart a powerful reaction of the sort seen in a reactor core - which would make it very hard to cool them effectively in the storage pool - Edano stated that this is not a realistic risk.